1. Field of the Invention
This invention relates to a plasma generation apparatus for generating a negative ion plasma using low-electron-energy plasma.
2. Description of the Related Art
In recent years there has been extensive use of plasmas in processes for manufacturing various semiconductor devices, liquid crystal displays, and solar cells, etc.
A dry etching process that employs plasma is used for example when subjecting thin films formed on the surfaces of substrates to dry etching. A specific example of such a dry etching process is seen, for instance, in the process of etching a silicon oxide film formed on a silicon semiconductor substrate using the action of active species or ions generated in a plasma.
Film forming processes that use plasmas are also employed in forming requisite thin films on the surfaces of substrates. A specific example of such a film forming process is seen, for instance in the plasma CVD (chemical vapor deposition) process for forming requisite thin films on the surfaces of substrates using a plasma-based vapor-phase reaction. A specific example of such a plasma CVD process would be a process for forming an inter-layer insulating film on a silicon semiconductor substrate.
In other words, in recent years, wiring interconnections have come to be implemented in multiple layers in conjunction with the higher integration of semiconductor devices. As a consequence, it has become necessary to provide insulating films between the wiring layers (inter-layer insulating films). CVD processes can be used in forming these inter-layer films. One such CVD process is the thermal CVD process, which is a process that uses heat as the necessary energy for activating the reaction. Specifically, reactive gas introduced into a process reaction chamber is made to react by the application of heat, thereby forming an inter-layer film.
This thermal CVD process requires a comparatively high temperature, however, which often results in problems in the devices. Recently, therefore, processes have come into use which employ a plasma as the activation energy. An example of a plasma used in such cams is a plasma generated through glow discharges.
Plasma CVD processes are also used in forming requisite thin films on substrates in solar cells.
In the dry etching processes that are typical of plasma processes, however, it is being demanded
(1) that the plasma can be generated uniformly at high density in order to cope with larger substrate areas and improved apparatus throughput, PA1 (2) that process precision and selectivity can be improved in order to cope with electronic device structure miniaturization and multi-layer implementation, and PA1 (3) that uniform plasma can be generated in order to reduce charge-up damage. PA1 (1) When forming contact holes by etching a fine silicon oxide film, selectivity for the substrate silicon declines. PA1 (2) When etching gate polysilicon electrodes, abnormal side etching develops due to electric charge accumulation. PA1 (3) Gate oxide film insulation damage occurs.
In order to answer to these demands, in recent years, various kinds of plasma generation apparatuses (plasma sources) art being developed. Examples of such high-density plasma generation apparatuses are the ECR (electron-cyclotron-resonance) type plasma generation apparatus, the inductively-coupled plasma generation apparatus (ICP generator), the micro-surface wave plasma generation apps, the helicon wave plasma generation apparatus, and the magnetron high-frequency discharge plasma generation apparatus.
Adequate plasma density can be achieved with these apparatuses. When it comes to plasma uniformity, however, as things stand now, adequate uniformity cannot be achieved when .phi. is in the 300 mm range.
In these apparatuses, moreover, it is demanded that the plasma electron temperature be kept low in order to suppress excessive dissociation in the process gas.
Nevertheless, high-density plasma generation apparatuses are now being developed for dry etching silicon oxide films. There are serious problems to overcome in these apparatuses, however, namely the accumulation of electrical charges on the substrate surface and the reduction in etching selectivity resulting from excessive dissociation in the gas.
In the etching processes that employ current high-density plasma generation apparatuses, the following problems are being faced.
It is believed that these phenomena occur because of the presence of many high-energy electrons in the plasma generated by low-pressure high-density plasma generation apparatuses. In other words, it is believed that these problems are produced because of the high temperatures of the plasma electrons. That is, when the plasma electron temperature is high, dissociation reactions proceed excessively in the plasma. Consequently, the radical species (CFx radicals, etc.) that are determinative of selectivity become few, or the sheath potential produced at the substrate sure (difference between mean potential in plasma space and substrate surface potential) becomes high. As a result, it is thought, charge accumulation becomes large due to the sheath potential distribution caused by substrate irregularities and the plasma density distribution, whereupon the phenomena noted above arc produced. By sheath potential here is meant the potential at the substrate surface relative to the mean potential in plasma space.
In view of the foregoing, there is a need to develop a method for keeping the electron temperature low in process plasmas. Two methods of achieving this are currently being considered, namely a pulse modulation plasma method and a grid control method.
By pulse modulation plasma method here is meant a method for generating plasmas having low electron temperature by intermittently supplying electric power for plasma generation to a plasma generation electrode. That is, by repeatedly executing and terminating power supply, plasma exhibiting low electron temperature is generated In other words, this is a method for generating a plasma of low electron temperature by subjecting the plasma generation power to a pulse modulation. The pulse signals used in this case are signals having a small pulse width of several tens of micrometers or so.
By employing this method, the electron temperature can be lowered while maintaining the plasma density to some degree. In more specific terms, the speed of plasma density decline when the power supply is terminated is slower than the speed of electron temperature decline. Thus, by repeatedly executing and terminating the power supply, the electron temperature can be lowered while maintaining some degree of plasma density.
This method can be applied to any of the high-density plasma generation apparatuses noted earlier. When that is done, however, the pulse modulation frequency at which optimum electron temperature is obtained differs from apparatus to apparatus. This is due to the differences in rise time and decay time in plasma density and electron temperature between the different apparatuses.
By grid control method is meant a method wherewith a plasma of low electron temperature is generated in a plasma diffusion region by using a grid to divide the region inside the vacuum vessel between a plasma generation region and a plasma diffusion region.
FIG. 17 is a diagram representing the configuration of a conventional plasma generation apparatus wherein the grid control method is adopted as the method of reducing electron temperature. This figure diagrams a representative case wherein the grid control method is applied to a magnetron high-frequency discharge plasma generation apparatus. In the figure, an example configuration is diagrammed for a substrate surface processing apparatus having such a plasma generation apparatus. In the figure, moreover, in the interest of diagrammatic clarity, hatching is used to indicate cross-sections only for some configurational elements.
In the apparatus diagrammed in the figure, a flat plate-shaped grid 42 is placed so that it lies perpendicular to the center axis Z of a vacuum vessel 41, thereby dividing the region inside the vacuum vessel 41, in the center axis Z direction, between a plasma generation region R1 and a plasma diffusion region R2, In other words, the diagrammed plasma generation apparatus is made so that, by placing the plate-form grid 42 so that it lies parallel with a substrate W, the region inside the vacuum vessel 41 is divided, in the center axis Z dimension, between the plasma generation region R1 and the plasma diffusion region P2.
However, when the discharge power becomes large in a plasma generation apparatus that uses the pulse modulation plasma method as the method for reducing the electron temperature, it becomes difficult to obtain low electron temperatures, which is a problem. This is so because, when the discharge power is great, it becomes difficult for the electron temperature to decline when the power supply is stopped.
With this apparatus, moreover, if the power supply stop time is lengthened in order to obtain plasma of low electron temperature, plasma generation efficiency declines, and substrate processing efficiency also declines. These are problems too.
These problems do not arise, however, with a plasma generation apparatus that employs the grid control method as the method of lowering the electron temperature. This is so because, with such an apparatus, nothing at all is done to the discharge power.
Nevertheless, there is also a problem with this apparatus in that, when the substrate dimension become large, the electron temperature distribution at the substrate surface sometimes becomes uneven.
More specifically, in this apparatus, the grid 42 is placed so that it is parallel to the substrate W. Thus, when the dimensions of the substrate W become large, the caliber of the grid 42 also becomes large. As a result, the grid 42 sometimes suffers deformation when heated by the plasma. When the grid 42 becomes deformed, the parallelism between the grid 42 and the substrate W collapses. When this happens, the electron temperature distribution at the surface of the substrate W becomes uneven.
Thereupon, an object of the present invention is to provide a plasma generation apparatus wherewith the electron temperature distribution at the surface of the process object can be prevented from becoming uneven even when the dimensions of that process object are large.
In order to resolve the problems noted in the foregoing, the plasma generation apparatus cited in claim 1 comprises: a tubes vacuum vessel, gas induction means, atmosphere exhausting means, a ring-shaped discharge electrode, discharge electrode power supply means, region division means, and electron temperature control means.
Here, the gas induction means functions to introduce discharge gas into the interior of the vacuum vessel The atmosphere exhausting means functions to exhaust the atmosphere in the interior of the vacuum vessel. The discharge electrode is provided concentrically with the vacuum vessel, and functions to generate plasma in the peripheral region of the vacuum vessel by causing the discharge gas to discharge. The discharge electrode power supply means fictions to supply electrical discharge power to the discharge electrode for causing the discharge gas to discharge.
The region division means covers the inside of the discharge electrode near the discharge electrode, thereby dividing the region inside the vacuum vessel, in a direction perpendicular to the center axis thereof, into a plasma generation region and a plasma diffusion region. These region division means comprises a tube-shaped wall having a plurality of electron passing holes therein. This wall is placed concentrically with the vacuum vessel so that it is positioned outside the position wherein the process object is placed. The electron temperature control means functions to control the electron temperature of the plasma in the plasma diffusion region.
As based on the apparatus cited in claim 1, the interior regions of the vacuum vessel can be divided into a plasma generation region and a plasma diffusion region in a direction perpendicular to the center axis of the vacuum vessel. Thus the strength of the wall can be prevented from declining even when the dimensions of the process object are large. As a result, it is possible to prevent the electron temperature distribution at the surface of the process object from becoming uneven due to deformation of the wall.
As based on this apparatus, moreover, the interior regions of the vacuum vessel can be divided between a plasma generation region and a plasma diffusion region, Thus it is possible to control the plasma in the plasma diffusion region without affecting the plasma in the plasma generation region. As a result, it is possible to enhance the controllability of the plasma in the plasma diffusion region.
As based on this apparatus, furthermore, the region division means are placed near the discharge electrode. This makes it possible to effectively control the electron temperature in the plasma in the plasma diffusion region.
As based on this apparatus, moreover, the region division means are positioned outside the process object. Thus the plasma density can be made uniform over the entire surface of the process object. As a result, plasma processing can be performed uniformly over the entire surface of the process object.
The plasma generation apparatus cited in claim 2 is the apparatus cited in claim 1, wherein, the wall exhibits electrical conductivity As based on the apparatus cited in claim 2, region division is accomplished using the wall that exhibits electrical conductivity. Thus the electron temperature of the plasma in the plasma diffusion region can be made lower than the electron temperature in the plasma generation region.
The plasma generation apparatus cited in claim 3 is the apparatus cited in claim 2, wherein the electron temperature control means comprises insulation means. This insulation means functions to electrically insulate the wall from a reference potential point.
As based on the apparatus cited in claim 3, the wall can be electrically insulated from a reference potential point. Thus it is possible to control the electrical characteristics of the wall. As a result, using this wall, it is possible to control the electron temperature of the plasma in the plasma diffusion region.
The plasma generation apparatus cited in claim 4 is the apparatus cited in claim 3, wherein the electron temperature control means comprises a capacitive element. This capacitive element is inserted between the wall and the reference potential point.
As based on the apparatus cited in claim 4, the high-frequency impedance of the wall can be made small, when that is the case, it is possible to suppress fluctuations in the wall potential, even when the potential in the plasma space fluctuates due to the application of high-frequency electric power to the discharge electrode. As a consequence, the sheath potential generated at the surface of the wall can be set to a desired potential. Thus it becomes possible to lower the electron temperature of the plasma in the plasma diffusion region to a rather low temperature.
The plasma generation apparatus cited in claim 5 is the apparatus cited in claim 3, the electron temperature control means comprises potential control means. This potential control means functions to control the potential of the wall As based on the apparatus cited in claim 5, the potential of the wall can be controlled. Thus the sheath potential generated at the surface of the wall can also be controlled. As a result, the electron temperature of the plasma in the plasma diffusion region can also be controlled.
The plasma generation apparatus cited in claim 6 is the apparatus cited in claim 3, the electron temperature control means comprises potential control leans and a capacitive element. The potential control means functions to control the wall potential. The capacitive element is inserted between the wall and a reference potential point.
As based on the apparatus cited in claim 6, the electron temperature of the plasma in the plasma diffusion region can be lowered to a rather low temperature, and this electron temperature can be controlled over a wide range.
The plasma generation apparatus cited in either claim 7 or claim 8 is the apparatus cited in either claim 4 or claim 6, wherein the capacitive element is a variable capacitive element.
As based on this plasma generation apparatus cited in either claim 7 or claim 8, the capacitance of the capacitive element can be controlled. Thus the high-frequency impedance of the wall can also be controlled. As a consequence, the sheath potential generated at the surface of the wall can also be controlled. Thus the electron temperature of the plasma in the plasma diffusion region can also be controlled.
The plasma generation apparatus cited in either claim 9 or claim 10 is the apparatus cited in either claim 5 or claim 6, wherein the potential control means controls the DC potential on the wall.
As based on the apparatus cited in either claim 9 or claim 10, the electron temperature of the plasma in the plasma diffusion region can be continuously controlled over a broad range.
The plasma generation apparatus cited in claim 11 is the apparatus cited in claim 1, wherein the electron temperature control means comprises area adjustment means. This area adjustment means functions to adjust the total area of the plurality of electron passing holes.
As based on the plasma generation apparatus cited in claim 11, the total area of the plurality of electron passing holes can be adjusted. Thus the sheath potential generated in the electron passing holes can also be controlled. As a consequence, the electron temperature of the plasma in the plasma diffusion region can also be controlled.
This area adjustment means may be, for example, means that adjusts the total area of the plurality of electron passing holes by adjusting the area of each electron passing hole, or this may be such as to adjust the total area by adjusting the number of electron passing holes.
The plasma generation apparatus cited in claim 12 is the apparatus cited in claim 1, wherein the wall is divided so that it comprises a plurality of small walls, and the electron temperature control means comprises interval control means capable of adjusting the intervals between the plurality of small walls.
As based on the apparatus cited in claim 12, the intervals between the plurality of small walls can be adjusted. Thus the potential barrier generated between the plurality of small walls can be controlled. As a consequence, the breadth of the electron energy distribution in the plasma diffusion region can also be controlled. Hence there is no need to bias the wall. As a result, in processes wherein reaction gasses are employed, the electron temperature of the plasma in the plasma diffusion region can be controlled even when an insulating film is formed on the surface of the wall.
The plasma generation apparatus cited in claim 13 is the apparatus cited in claim 1, wherein the temperature control means comprises interval adjustment means. This interval adjustment means can adjust the interval between the wall and the discharge electrode.
As based on the apparatus cited in claim 13, the interval between the wall and the discharge electrode can be adjusted. Thus the sheath potential generated at the surface of the wall can be controlled. As a consequence, the potential barrier formed by the wall can be controlled also. Thus the energy possessed by the electrons which can cross over this potential barrier varies. As a result, the electron temperature of the plasma in the plasma diffusion region can be controlled.
As based on this apparatus, moreover, plasma generation efficiency can be prevented from declining arid the apparatus can be prevented from becoming large. More specifically, when the interval between the wall and the discharge electrode becomes too small, discharges are generated between the wall and the discharge electrode. This causes plasma generation efficiency to decline. Conversely, when that interval is too large, the apparatus becomes larger. Hence it is evident that, by being, able to adjust the interval between the wall and the discharge electrode, plasma generation efficiency can be prevented from declining and the apparatus can be prevented from becoming large.
The plasma generation apparatus cited in claim 14 is the apparatus cited in claim 1, wherein the interval between the wall and the discharge electrode is set to an interval wherewith no abnormal discharge will be generated between the two.
As based on the apparatus cited in claim 14, abnormal discharges can be prevented from being generated between the wall and the discharge electrode. Hence plasma generation efficiency can be enhanced.
The plasma generation apparatus cited in claim 15 is the apparatus cited in claim 1, wherein the wall is placed so as to be parallel to the center axis of the vacuum vessel.
As based on the apparatus cited in claim 14, the wall can be formed easily. The plasma generation apparatus cited in claim 16 is the apparatus cited in claim 1 wherein the wall is tilted relative to the center axis of the vacuum vessel so that it can be oriented toward the process object.
As based on the apparatus cited in claim 16, the plasma density at the surface of the process object can be raised. Hence the process object processing efficiency can be enhanced.
The plasma generation apparatus cited in claim 17 is the apparatus cited in claim 1, wherein the wall is covered by a dielectric.
As based on the apparatus cited in claim 17, it is possible to prevent metallic impurities from coming out from the surface of the wall due to the interaction between the wall and plasma.
The plasma generation apparatus cited in claim 18 is the apparatus cited in claim 1, wherein the region division means comprises a first and a second ring-shaped partition panel. The outer edge of the first partition panel is fixed to the inner wall side of the vacuum vessel. To the inner edge of this partition panel is fixed one end of the wall. The second partition panel is deployed so that, together with the first partition panel, it sandwiches the discharge electrode. The outer edge of this partition panel is fixed to the inner wall side of the vacuum vessel. To the inner edge of this partition panel is fixed the other end of the wall.
As based on the apparatus cited in claim 18, it is easy to configure the region division means that covers the discharge electrode from the inside thereof The plasma generation apparatus cited in claim 19 is the apparatus cited in claim 1, wherein a plurality of electron passing holes are arrayed in a lattice pattern.
As based on the apparatus cited in claim 19, it is easy to configure the wall having electron passing holes.